Single nucleotide polymorphisms are specific nucleotide locations in the genome that vary in sequence from person to person. A SNP exists on average every 300 bases, for an estimated total of 10 million SNPs in the human genome. SNPs can occur in genes or in non-coding regions, and may be direct causes of genetic mutations or may serve as benign markers for linkage and association studies.
The cataloging of SNPs has played a major role in the explosion of genome-wide association studies (GWAS) over the past decade. These high-throughput experiments have enabled the systematic analysis of hundreds of thousands of SNPs with thousands of individuals with different complex disorders and traits. GWAS have identified several thousand SNPs that are associated with complex disorders including type 2 diabetes, Crohn’s disease, and mental illness.